The shape of the PC market is changing, due at least in part to the way PCs themselves are changing shape.

According to recent research from Gartner, one of the fastest-growing device categories has achieved enough momentum to earn its own name: the new Premium Ultramobile category includes Apple’s MacBook Air, Microsoft’s Surface Pro, and other lightweight devices that have the full power of a PC or Mac inside but are optimized for mobility.

By contrast, Chromebooks and Windows RT-based devices like the Surface 2 are struggling.

Gartner numbers predict big gains for premium devices like the Surface Pro and MacBook Air

Demand for traditional desktop and notebook PCs is declining slowly, but that decline is being more than offset by strong growth in what Gartner calls the Premium Ultramobile category.

Here’s the formal definition (emphasis added):

Premium ultramobiles extend the notebook usage model toward the tablet by refinement of physical characteristics, such as less weight, smaller size and smaller screen size (to enable easier portable usage), and instant-on. Premium ultramobiles typically weigh 1.6 kilograms (kg) and less. They are user-interface-optimized for media consumption, while retaining capabilities for full-scale data processing. Such a device will provide a good productivity and content creation capability compared with basic ultramobiles. It will, therefore, be an alternative to a notebook, dependent on the trade-offs that a user wishes to make between the characteristics of devices and their expected usage pattern. This category includes Microsoft's Windows 8 Intel x86 products and Apple's MacBook Air.

That reference to “Microsoft’s Windows 8 Intel x86 products” currently includes only the Surface Pro line, which is now in its third iteration. And in addition to MacBook Airs (but not MacBook Pros) this category also includes high-end Ultrabooks and hybrids from Lenovo, Dell, HP, Acer, and other PC OEMs.

Gartner first broke out sales numbers for the Ultramobile category beginning in 2012, when it reported 9.8 million units, including MacBook Airs and some early Windows hybrid PCs. In 2013 total shipments in the Premium Ultramobiles category grew to 21.5 million units (with another 3 million units split off into the Other Ultramobiles, Hybrid and Clamshell cagtegory), and the research firm predicts that this segment of the market will grow by 50 percent in 2014 and then increase by 70 percent the following year, with a total of 55 million Premium Ultramobile devices expected to ship in 2015.

To put that number in perspective, Apple sold a total of 17.2 million Macs of all configurations in 2013 and 17 million in 2012. Those totals include iMacs, MacBook Pros, Mac Minis, and Mac Pros, which are counted in the Traditional PCs category.

Or, to put it another way: The worldwide market for all x86-based PCs and Macs will be roughly the same size in 2015 as it was in 2013. But super-lightweight premium PCs optimized for mobility will make up more than 17 percent of that market next year, compared to less than 7 percent in 2013. That's a sizable shift.

If Gartner’s predictions are accurate, the installed base of Premium Ultramobile devices that are less than three years old will exceed 100 million by the end of 2015. Of that total, Windows-based devices should make up roughly 80 percent, with the remainder mostly consisting of MacBook Airs.

Meanwhile, what’s happening to ultramobile devices that aren’t full PCs or Macs? Gartner now breaks those into two subcategories: Basic Ultramobiles and Utility Ultramobiles. The hybrid and clamshell devices in this category include Chromebooks (considered Basic and not Premium devices “because their functionality is considerably limited when not connected to the Internet”). The category also includes Windows RT devices such as the Surface 2 and Nokia Lumia 2520.

Gartner projects that a total of only 16 million Chromebooks, Windows RT machines, and other hybrid and clamshell devices in this group will ship in the three-year period from 2013 through 2015. That number is dwarfed by the 108 million Premium Ultramobile devices expected to ship in the same period.

(On a side note, the most recent browser usage figures from Net Applications and StatCounter confirm the Chromebook's relative rarity. Although ZDNet's coverage of these numbers last week focused on flat growth for Windows 8.x, it's worth noting that Chrome OS hasn't even achieved a blip on the Net Applications/NetMarketShare radar screen. A spokesman for the company confirms that Chrome OS usage on its network has not risen to the level of 0.1 percent of all PCs worldwide, the minimum required to earn a mention in the monthly reports.)